Blood Vessels Lab

Learning Objectives

Distinguish the different blood vessels at the light and electron microscope levels, taking note of the diameter of the vessel lumen in relation to vessel wall thickness

Describe the major features of blood vessels of different diameters, including the predominant wall-forming layer and the functional importance of this layer

Understand the structural variations in capillary endothelium and their implications for capillary permeability

Identify some important pathological examples involving blood vessels

Keywords

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Pre-Lab Reading

Layers of the Vascular Wall

The vascular system has a common histological organization that appears with particular clarity at the level of the large vessels. The tissue components (endothelium, smooth muscle, elastic elements and connective tissue) that form the vascular walls are arranged in concentric layers. Under the influence of local functional factors, this layered structure has undergone segmental differentiations that characterize each type of vessel. For descriptive purposes the concentric layers have been classified as three tunics as follows (from the lumen outward):

The tunica intima comprises the endothelium, the adjacent basement membrane, the subendothelial connective tissue, and the internal elastic lamina; in smaller vessels, pericytes appear between layers of the split basement membrane of the endothelium.

The tunica media is composed of smooth muscle cells, elastic lamellae including the external elastic lamina and collagen fibers.

The tunica adventitia contains connective tissue, a few cells, macrophages, mast cells, fibroblasts, and the nerves and vessels that supply the vascular wall.

As a result of segmental differentiations, some features of these tunics are accentuated, others are reduced or omitted, or other additional structures are introduced. Accordingly, the detailed organization of the vascular wall varies characteristically from segment to segment.

Classification of Vascular Segments

Blood vessels are divided into two broad categories: macrovasculature and microvasculature. The macrovasculature is composed of those blood vessels that can be seen with the naked eye. The microvasculature is composed of blood vessels that are smaller than 100 microns may only be seen through the microscope. While the vessels of the macrovasculature appear as isolated anatomical entities, the microvasculature appears structurally and functionally as part of the tissue it supplies.

Elastic arteries are primarily represented by the large vessels emerging from the heart ventricles, such as the aorta and the pulmonary artery. Characteristic for these vessels are numerous concentric elastic lamellae of the tunica media interspersed with bundles of smooth muscle cells. Elastin can be stained with special stains. Elastic fibers in the media allow for the maintenance of blood pressure through the expansion and contraction of the vessel walls. The vasa vasorum, a network of small vessels that supplies the cells of larger vessels, is present in their adventitia and outer part of the media.

Muscular arteries follow the elastic arteries. Through the controlled contraction of their walls, these arteries distribute the blood to different parts of the body according to regional needs. The media of muscular arteries is composed of many concentric layers of smooth muscle cells arranged in a low angle helix, interspersed with less frequent and sometimes discontinuous elastic lamellae. The internal elastica marks the conventional boundary between intima and media. The external elastica separates the media from the adventitia. The media and adventitia of muscular arteries are approximately equal in thickness.

Arterioles contain an internal elastic lamina and one or two layers of smooth muscle cells. There is no external elastic lamina, and the adventitia consists of a thin layer of collagen and isolated elastic fibers. By the contraction of their muscle fibers, the arterioles generate the "peripheral resistance" that reduces the blood pressure at the periphery, and thereby protects the capillaries and venules.

Capillaries are vessels of small diameter (4 to 10 microns) whose wall is reduced to an attenuated endothelium surrounded by a basement membrane, a few pericytes, and connective tissue. The narrow capillary lumen allows passage of red blood cells in single file. At the level of the electron microscope, three different types of capillaries can be resolved based on the morphology of their endothelial layer:

Capillaries with a continuous endothelium are less permeable and are present in muscles, lung, connective tissue, and skin.

Capillaries with a fenestrated endothelium have gaps between endothelial cells, but the basement membrane is still continuous. These are found in the renal glomeruli, endocrine glands, intestinal villi, and exocrine pancreas.

Capillaries with a discontinuous endothelium have large gaps between cells and a discontinuous basement membrane. Such capillaries are called sinusoids and are found in the liver and in blood-forming and lymphoid organs. These capillaries play an important role in blood/interstitial fluid exchanges and explain the high degree of capillary permeability for water and water-soluble molecules, as well as plasma protein and hormones.

Venules are tubes of endothelium. Small venules (up to 40-50 µm diameter) are surrounded by pericytes, contractile cells with long, branching processes that are involved in the control of blood as it flows through the microvasculature. Large venules (50-100 µm diameter) are surrounded by one or two layers of smooth muscle cells. Beyond the pericytes and the smooth muscle cells is a thin layer of connective tissue. The venular endothelium has labile junctions which "open" in inflammatory reactions under the influence of histamine, serotonin, bradykinin and other agents. The result is increased permeability and local swelling. Diapedesis, the exit of leukocytes from the vasculature, occurs also at this level of the microvasculature. You will study this process in more detail in Pathology.

Veins of small and medium size are characterized by a thin media containing only a few layers of smooth muscle cells. These vessels have a much thicker adventitia composed of collagen and occasionally some longitudinal smooth muscle fibers. In general, veins are larger in diameter and have thinner walls than arteries. The tunica adventitia makes up the greater part of the venous wall of large veins and is usually considerably thicker than the tunica media.

Special Vessels

Other variations in the structure of blood vessels occur in certain organs in response to special functional and anatomical conditions. Some examples of special vessels include:

Cerebral arteries and veins: These arteries are rather thin-walled for their caliber, with a well-developed internal elastica and virtually no elastic fibers in the rest of the vascular wall. The veins have a thin wall devoid of smooth muscle cells.

Pulmonary arteries and veins: These arteries have thin walls as a result of a significant reduction in both muscular and elastic elements, while the veins have a well-developed media of smooth muscle cells.

Umbilical vessels: These arteries have two layers of smooth muscle cells without a prominent internal elastica or adventitia. The vein has a thick muscular wall with two to three muscle layers.

Portal systems: A portal system is one in which two capillary networks are connected in series by an arteriole or venule. In the kidney, an arterial portal system is present at the level of the glomeruli. A venous portal system exists in the liver.

Pre-Lab Quiz

List, in order, the specific segmental variations of blood vessels that a red blood cell
would travel through on a continuous circuit beginning in the left ventricle and ending in the right
atrium of the heart.

The blood-brain barrier (BBB) separates the tissues of the brain and spinal cord from
direct exposure to the contents of the bloodstream. What type of endothelium would you expect to see
in the capillaries of the BBB? Where else would you find this endothelium?

Answer: The blood-brain barrier has a continuous endothelium, which can also be found in the muscles, lung, and skin. Gases diffuse across the endothelium, but most other substances must be transported.

As you move through the arterial system from large to small arteries, what change would
you expect to see in the relative amounts of elastic tissue and smooth muscle cells?

Answer: You would expect to see a decrease in the amount of elastic tissue and an increase in the amount of smooth muscle cells.

As you move from arteries to veins, what change would you expect to see in the relative
sizes of the tunica intima, media, and adventitia?

Answer: The intima remains the same, the media decreases in size, and the adventitia increases in size as you move from the arteries to the veins.

Slides

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What changes would you expect to see in the relative sizes of the tunica intima,
media, and adventitia of an elastic artery as a patient develops atherosclerosis?

Answer: The intima with increase in size because of the growth of the plaque and infiltration of macrophages, fibroblasts, and smooth muscle cells. The media will decrease in size as the intima grows. The adventitia may remain the same, or may also decrease in size.